"I'm not sure how serious it is," McDermott said. "He's obviously done something, but it's pointless speculating.
"He tried to run it off so you'd imagine it's not really bad. But there's something in there and he's got a brace on. We'll know more tomorrow."
Tries from Brad Singleton and Ryan Hall put the champions into a 10-6 lead early in the second half, but Warrington finished the stronger side and clinched victory with winger Kevin Penny's 68th-minute try.
McDermott was far from downhearted, though, and did not believe the loss of McGuire was pivotal.
"I thought while Danny was on there, we looked to lack a bit of spite on attack so I wouldn't say a deciding factor," he said.
"It was a tough game to start with. There were three or four things that wanted to be good at and we were so there were more positives than negatives.
"Both teams defended well, but we probably lacked a bit of fluency and cohesion when we attacked their try-line."
Leeds had a chance to secure a draw, but full-back Zak Hardaker skewed a penalty kick at goal four minutes from the end, awarded after replacement half-back Jordan Lilley was obstructed as he chased his own grubber kick.
"People miss goals," McDermott said. "It will happen. It was not a sitter, but one of the easier ones to get."